Hi all, I have Lync2013 (collocated server) with edge and Cisco UC500 configured as a trunk with media bypass. 

It takes rather long up to 20 seconds for the called phone to starts ringing. 

Any ideas what to look at?

There are a few items that could be affecting it.  Clients may not have access to the edge's internal interface on port TCP/443 and UDP/3478.   The routing in both directions must be set up as well.  

Also, see if you can disable any endpoint protection, malware, personal firewall, or AV software that may exist on the machine, they may be trying to inspect traffic. 

Does it make a difference without media bypass?